Advances in satellite alteration mineral mapping for gold

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by Gerry Mitchell, P. Geo, PhotoSat President

Vancouver BC, May 2018

Zones of opal, chalcedony and other hydrous silica minerals at Cuprite, Nevada as detected by the short-wave infrared camera

on the WorldView-3 satellite. These hydrous silica zones can be important indicators of underlying gold and silver deposits.

Advances in satellite alteration mineral

mapping for gold and silver deposits

Higher spectral- and spatial-resolution data and

better algorithms for increasingly detailed predictive mapping

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PhotoSat, Engineering-grade Satellite Surveying | www.photosat.ca | 2018

Many new

satellite

exploration tools

were developed

since the last

exploration cycle

The new

WorldView-3

(WV3) satellite

photos became

commercially

available in 2015

Figure 1. Silica

mapping in the

Cuprite, Nevada

using ASTER and

WorldView-3

satellite photos. The

WorldView-3 clearly

identifies only the

opal and chalcedony

in the Cuprite

epithermal

alteration zone

while the ASTER

shows all the areas

of high silica.

ew satellite alteration mineral mapping tools and

capabilities have been developed since the last mining

exploration cycle. The last cycle ended for most of us in

2012. Since then there has been very little funding for

mining exploration. Most of these new capabilities are

awaiting their first application in many promising

exploration districts.

The WorldView-3 (WV3) satellite was launched in

September 2014. The WV3 photos became commercially

available in early 2015. WV3 enables much better detection

and mapping of alteration minerals associated with gold

and silver deposits than did previous satellite systems

(Figure 1).

N

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The WV3 short-

wave infrared

camera has over

15 times better

spatial resolution

than the ASTER

SWIR camera

Worldview-3

maps “new”

alteration

minerals

WordlView-3 can reliably detect much smaller zones of

alteration minerals than any previous commercially

available satellite system (Figures 1, 2). Prior to the WV3

short-wave infrared (SWIR) photos, the best satellite SWIR

photos were those from the ASTER camera onboard the

Terra satellite. The SWIR camera on the WV3 satellite has

over 15 times the spatial resolution of the ASTER SWIR

camera. WV3 SWIR pixels are 56 m2 whereas ASTER SWIR

pixels cover 900 m2.

WV3 satellite photos can reliably map minerals that we

could not confidently identify with previous satellite

systems. The most important of these for gold and silver

exploration are probably the hydrous silica minerals opal

and chalcedony. These minerals occur in the uppermost

levels of epithermal gold and silver deposits (Figure 3).

Figure 2. Hydroxyl alteration minerals at the Ixtaca deposit in Puebla, Mexico.

Due to differences in spatial resolution, the WorldView-3 short-wave infrared

photos can detect smaller mineral alteration zones with greater certainty and

show far more detail than ASTER photos.

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Figure 3. Geological model of mineral alteration zones for the epithermal

silver deposit at Juanicipio, Zacatecas, Mexico. Model developed by Peter

Megaw, modified from Buchanan (1981) and Simmons (1991). Peter

Megaw and his team at Mag Silver used this model to make the amazing

blind* discovery of the Juanicipio silver deposit discovery in 2003. Legend:

(1) Siliceous residue: opal, chalcedony, cinnabar, pyrite, specularite.

(2) Advanced argillic alteration: ammonium alunite, kaolinite, buddingtonite.

(3) Silicification: usually with adularia.

(4) Propyllitic alteration: chlorite, epidote, calcite, pyrite, montmorillonite. (5) Adularization: albite increases below the boiling level.

*There was absolutely no indication of gold or silver at surface above the

Juanicipio deposit, making it a “blind” discovery. The ore zone is 450m

below the ground surface. It was discovered through geological reasoning,

interpretation, perseverance and courage.

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WV3 has better

definition of the

minerals

associated with

gold and silver

deposits than

ASTER

Alteration

minerals

detectable with

WV3 and/or

ASTER

Many

opportunities to

be first with WV3

alteration mineral

mapping

The importance of being able to map the hydrous silica

zones is shown in Figure 1. At Cuprite, Nevada the WV3

hydrous silica clearly shows the heart of the epithermal

alteration zone. By comparison, ASTER shows all of the

silica zones, not differentiating the epithermal hydrous

silica zone from other areas of silica.

The WV3 SWIR camera produces much higher quality

photos than the ASTER SWIR camera. This enables better

discrimination of the clay, mica, hydrous iron oxide, and

ammonium feldspar alteration minerals that are guides for

exploration and development on many gold and silver

projects.

Table 1 lists alteration minerals for epithermal gold and

silver deposits detectable with the WV3 and/or ASTER. The

listed minerals are often used as prospecting guides. A

conceptual model of an epithermal deposit showing the

zones of different alteration minerals is shown in Figure 3.

Since the WV3 satellite photos first became available in

2015, international investment in mineral exploration has

been at an historical low. Consequently, WV3 alteration

mineral mapping has not yet been applied to many highly

prospective epithermal gold and silver mineral belts.

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Table 1. Alteration

minerals for

epithermal gold and

silver deposits

detectable with

WV3 and/or ASTER.

Most of these

minerals are best

detected on photos

from the short-

wave infrared

(SWIR) cameras.

These cameras

detect reflected

light with

wavelengths

between 1,100 and

2,500 nanometers

(Figure 7). Figure 7

shows the

wavelength of

WorldView-3 SWIR

band 8, and ASTER

SWIR band 6.

Clay

Alunite (Na,K)Al3(SO4)2(OH)6

Kaolinite Al2Si2O5(OH)4

Montmorillonite (Na,Ca) 0.3 (Al,Mg)2Si4O10(OH)2•n(H2O)

Micas

Muscovite KAl2(Si3Al)O10(OH,F)2

Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]

Paragonite NaAl2(AlSi3O10)(OH)2

Sericite Sericite is not a specific mineral. It is a

hydrothermal alteration of orthoclase or

plagioclase feldspars. It may consist of

each, or a combination of, Muscovite,

Illite, and Paragonite.

Ammonium Feldspar

Buddingtonite NH4AlSi3O8

Silica

Quartz SiO2

Hydrous Silica

Opal, SiO2•n(H2O) Chalcedony SiO2•n(H2O)

Ferric Hydrous Iron Oxides

Goethite Fe3+O(OH) Jarosite KFe3+3(SO4)2(OH)6

Ferric Iron Oxide

Hematite Fe3+2O3

There are other alteration minerals that are important for

gold and silver prospecting that are beyond the detection

capacity of the ASTER and WV3 cameras.

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USGS High

Resolution

Spectral Library

The reflectance spectra of mineral samples form the basis

for much of the improvement in satellite mineral mapping.

The US Geological Survey has an extensive publicly

available library of mineral reflectance spectra: USGS

Spectral Library.

Mineral reflectance spectra from this library is used to

determine the expected spectral signatures of different

minerals in multiband satellite photos. Figure 5 shows the

expected spectral signatures of the clay alteration minerals

Kaolinite and Alunite on WV3 visible and near-infrared and

short-wave-infrared satellite photos. WV3 is much better at

differentiating the reflectance responses of kaolinite and

alunite than previous satellite systems.

Figure 4. WorldView-3 indications of kaolinite and alunite alteration at the

Juanicipio silver deposit, Zacatecas, Mexico. Worldview-3 is much better at

differentiating kaolinite and alunite than previous satellite systems. The silver

veins are more than 400m below the current ground surface.

https://speclab.cr.usgs.gov/spectral-lib.html

https://speclab.cr.usgs.gov/spectral-lib.html

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Field portable

reflectance

spectrometers

The Worldview-3

and ASTER

satellite cameras

Portable, field reflectance spectrometers can provide at-

surface spectral signatures for the area of the satellite

alteration mineral mapping. This enables adjustment of the

data processing to improve the accuracy and reliability of

alteration mineral mapping.

Almost all satellite-based alteration-mineral mapping is

carried out using photos from the Worldview-3 and ASTER

cameras. The Worldview-3 satellite became operational in

2015. The ASTER camera operated on the NASA Terra

satellite. The ASTER camera collected a global database of

multispectral satellite photos from February 2000 to April

2008.

Figure 5. USGS laboratory and WorlView-3 reflectance spectra for Kaolinite

and Alunite. Note the remarkable overlap between the two. Also note the

very different spectral responses between Kaolinite and Alunite in WV3 SWIR

bands B5, B6 and B7.

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Figure 6. ASTER camera on the Terra satellite on the left; Worldview-3 satellite

on the right. The ASTER has 5 visible and near infrared bands (VNIR), 6 short-

wave infrared bands (SWIR), and 5 thermal infrared bands (TIR). The Worldview-

3 has 8 VNIR bands, 8 SWIR bands, and no TIR bands.

Figure 7. Spectral and spatial characteristics of the WorldView-3 and ASTER

satellite cameras.

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Worldview-3 is

the only

operating

commercial

satellite with a

multiband SWIR

camera

ASTER alteration

mineral mapping

is still the best

technology for

regional

prospecting of

thousands of

square kilometers

The 8-band, 7.5m ground-resolution WV3 SWIR camera is

currently the only satellite camera in orbit with a multiband

short-wave infrared (SWIR) spectral range. The WV3 SWIR

satellite photos have better signal-to-noise ratio and a

wider dynamic range than any previous commercially

available SWIR satellite photos. The spectral and spatial

characteristics of the WV3 satellite camera are shown in

Figure 7.

The global database of ASTER satellite photos collected by

NASA and JAXA between 2000 and 2008 is still the best

data for alteration mineral studies of thousands of square

kilometers, covering large portions of mineral belts.

Figures 8 and 9 show examples of gold deposits with

distinctive alteration mineral anomalies on ASTER satellite

photos that were collected before the discovery of the

deposits.

Figure 8. ASTER silica and hydroxyl alteration over the El Sauzal gold mine in

western Chihuahua, Mexico. This ASTER photo predates the discovery of the

mine. Red represents strong, probable alteration. Blue represents weak, possible,

alteration.

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Advances in

satellite image

processing

knowledge and

methods are

improving WV3

and ASTER

alteration mineral

mapping

The continuing advances in computer processing capacity

are significantly improving our ability to identify false-

positive alteration-mineral responses on the WV3 and

ASTER satellite photos. This enables us to identify more

subtle alteration zones and map more effectively in

partially vegetated areas. We expect to see significant

advances in the near future from the application of

Convolutional Neural Networks (a.k.a. Deep Learning).

Figure 9. Hydroxyl mineral alteration zones on an ASTER photo of the

Ixtaca gold deposit, Puebla Mexico. Red represents strong, probable

alteration. Blue represents weak, possible, alteration.

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References:

Megaw Peter K.M., 2010 Discovery of the Silver-Rich

Juanicipio-Valdecañas Vein Zone, Western Fresnillo District,

Zacatecas, Mexico, Society of Economic Geologists,

Inc.Special Publication 15, pp. 119–132

Swayze, G.A., R.N. Clark, A.F.H. Goetz, K.E. Livo, G.N.

Breit, F.A. Kruse, S.J. Stutley, L.W. Snee, H.A. Lowers, J.L.

Post, R.E. Stoffregen, and R.P. Ashley, 2014, Mapping

advanced argillic alteration at Cuprite, Nevada using

imaging spectroscopy: Economic Geology, v. 109, no. 5, p.

1179-1221. doi:10.2113/econgeo.109.5.1179

Fred A. Kruse, William M. Baugh, and Sandra L. Perry,

2015, Validation of DigitalGlobe WorldView-3 Earth

imaging,satellite shortwave infrared bands for mineral

mapping: Journal of Applied Remote Sensing 096044-1

Vol. 9, 2015

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Advances in satellite alteration mineral mapping for gold